1. Technical Field
The present invention relates in general to pointing devices for data processing systems and in particular to trackpoint-type pointing devices. Still more particularly, the present invention relates to construction and operation of a capacitive trackpoint-type pointing device for use with data processing systems.
2. Description of the Related Art
Data processing systems with operating systems or applications employing a graphical user interface (GUI) typically include a pointing device such as a mouse, trackball, or touch pad. Using a pointing device, a user directs movement of a pointer icon (or "cursor," any graphical object utilized to point to a particular location in a display including, for example, a figure in a computer game) across the user interface by performing corresponding manual movements on the pointing device, for example by sliding the mouse, rolling the ball of the trackball, or moving across the surface of the touch pad. One or more buttons on the pointing device may be actuated to select items within the interface, such as icons representing applications or operating system controls.
One particular example of a pointing device employed in data processing systems is IBM Corporation's TrackPoint.RTM. pointing device, a trackpoint-type device similar to those frequently mounted in the keyboards of laptop or notebook data processing systems. A Trackpoint-type device includes a button-like structure (or "post") resembling a pencil eraser protruding from between the keys in a keyboard or from the surface of the structure incorporating the pointing device. The Trackpoint-type device eliminates the need for a flat working surface on which the user must manipulate a mouse.
A conventional implementation of a Trackpoint-type pointing device includes resistive strain gauge sensors and a post serving as a lever arm. The resistance of the strain gauges vary depending on the force applied by the use to the lever arm via the button-like structure. By manipulating the post, the user flexes the strain gauges. Small analog signals reflecting the resistance of the strain gauges are interpreted by on-board software and translated to emulate a displacement as an output. The displacement is typically calculated as a function of the duration for which the user flexes the strain gauges, and the cursor on the display is moved accordingly.
The strain gauges of a conventional Trackpoint-type pointing device must be individually trimmed during manufacture to match their outputs. The manufacturing and trimming of the strain gauges, combined with the small analog signal they produce, contribute to the cost of the sensor and the associated electronics. Additionally, the small full-scale magnitude of the resistive signal change is a burden for the data acquisition system which processes the strain gauge signals and translate flex into cursor movement signals.
One approach to overcoming the design and manufacturing issues associated with resistive Trackpoint-type pointing devices, described in the related applications, has been to employ capacitive sensors to measure manipulation of the post by a user. The capacitive type Trackpoint device is less expensive and easier to manufacture, provides a larger magnitude full-scale signal corresponding the user manipulations of the post, and provides an equally effective basis for translating user manipulation into equivalent displacement signals.
Both the resistive and capacitive Trackpoint-type devices typically employ "quadrature detection," in which four sensors are provided to measure forces applied normally in any lateral direction. Each sensor detects lateral movement of the post along an X or Y axis normal to the post. For capacitive Trackpoint-type devices, lateral force on the post translates to moving capacitive plates of a sensor closer together, altering the capacitance of a sensor. Axial force (Z axis) on the post, which also moves the sensor's capacitive plates closer together, may be compensated for by offsetting one sensor's change in capacitance by the change in capacitance of the opposite sensor.
In the capacitive Trackpoint-type device, which employs four capacitors to sense force in different directions, it would be desirable to use capacitors as large as possible. It would further be advantageous to have a capacitive Trackpoint-type device which utilizes as little power as possible.